mtd: chips: Use SPDX license headers

Some of the files in mtd/chips do not have a SPDX license
header, presumably because the text string "GPL'd" didn't
parse with Thomas rulesets for magic license tagging.

Fix this, the code is initially from RedHat which clearly
targeted the Linux kernel and intended it to be GPLv2.

In any case the original author appears to be David
Woodhouse who can then confirm this.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230603185200.3571174-1-linus.walleij@linaro.org